How Do I Patch Tile Backer Board?

We are tiling our tub surround with ceramic over tile backer board. We had to pull off the bullnose down the front edges because it was put up wrong, and it damaged the backer board in spots when prying it off. Can we use some kind of compound to "fill in" those spots where the backer board was ripped off before re-installing he bullnose tiles? Thanks for any advice.